1. Field of the Invention
The present invention is directed to the field of pulse width modulation techniques and more specifically to method and apparatus for adjusting appropriate modulation duty cycles when applied to devices such as electronic displays to obtain predetermined brightness levels.
2. Description of the Prior Art
Conventional approaches to control the brightness levels of electronic displays, such as vacuum fluorescent displays, are generally classified into two types.
A first type of approach involves varying the voltage or current levels applied to the display; and a second type of approach involves varying the duty factor.
The controlling of display brightness by varying the voltage or current applied thereto is generally recognized as being less acceptable, due to the non-uniform brightness that occurs at low levels and the high power requirements for the drive circuitry that must operate in a linear mode.
The varying of the duty factor, which is defined as that percentage of time the display is activated during each cycle period, is referred to as pulse width modulation and is more generally accepted as a preferable approach for controlling the brightness of an electronic display, since the problems of non-uniform brightness and high power requirements are eliminated. However, conventional pulse width modulation is limited in the number of selectable brightness level steps by the number of clock pulses present in a single cycle period. For instance, in a display system which employs "n" clock pulses per cycle period, the number of brightness level steps that may be selected is limited to "n".
Another disadvantage of conventional pulse width modulation is due to the fact that the step sizes are of equal magnitude and provide for activation of an associated display over a linear range. The linearly reduced brightness level of a display in equal brightness steps is perceived by the human eye as being a nonlinear function. Little change of brightness is perceived as the display is dimmed, until the lower end of the range is reached; and a large change in the brightness level steps is perceived at the dimmest levels.